This invention relates generally to the measurement of dynamic arc behaviour in a DC plasma arc furnace.
Effective operation of a DC arc furnace is based on the presence of an open, plasma arc above a molten bath of process material. The open arc which is established in a gaseous medium above the molten bath, is a primary heating and stirring element in the furnace and is critical to the proper functioning of the furnace.
A plasma arc comprises a high temperature, high velocity jet of ionised gas which extends between an electrode tip inside the furnace, and an upper surface of the molten bath. The arc is highly conductive and completes a non-linear electric circuit between a cathode and an anode of a DC source used to supply energy to the furnace.
The furnace includes a vessel, which forms an operating volume. A process material is contained in a lower part of the operating volume. Harsh conditions prevail inside the operating volume and visibility, for practical purposes, is generally zero. It can therefore occur that a process is operated with the electrode tip in direct contact with the molten bath. In this event the arc is extinguished and power is then supplied to the furnace by resistance heating of the molten bath. This is undesirable for it negates many of the advantage of open arc operation. It is desirable therefore to be able to establish whether an arc is present or absent in the electrical circuit of the furnace.
The environment around furnace is typically hot, dusty, inhospitable and prone to electromagnetic interference. An interior of the furnace vessel poses extreme challenges for the prevailing temperature is normally above 1500° C. and there are large quantities of dust and fumes present together with a high level of ultraviolet radiation. Measurement devices, such as furnace probe cameras, ultraviolet detectors, vibration transducers and microphones which can function under these conditions are costly, normally cannot be operated continuously, and usually require significant and expensive care and maintenance to ensure a degree of reliable operation.
Various techniques have been proposed in the prior art for measuring the stability of an arc in a furnace. Citations of relevance include U.S. Pat. No. 3,767,832, U.S. Pat. No. 4,476,565, U.S. Pat. No. 4,852,119, U.S. Pat. No. 5,050,185, U.S. Pat. No. 5,351,267, U.S. Pat. No. 5,533,044, U.S. Pat. No. 8,410,800 and US2009/0232181.
U.S. Pat. No. 3,767,832 is based on detecting a current increase which occurs when an electrode contacts metal in a vessel. U.S. Pat. No. 4,476,565 relies on detecting electromagnetic emissions which are related to the position of an electrode relative to a metal bath.
U.S. Pat. No. 4,852,119 describes a technique wherein harmonics are separated from a fundamental frequency and a comparison between two resultant signals is used as a means of quantifying the degree of arcing. The approach applies to an AC furnace and would not work in a DC arc furnace because there is no fundamental frequency in a DC furnace.
U.S. Pat. No. 5,050,185 states that increased foaming of slag in a furnace causes a reduction of harmonics in the arc current and that this is reflected in the energy levels of signals produced by suitable band pass filters.
U.S. Pat. No. 5,533,044 is based on detecting the field strength of an arc.
US2009/0232181 relates that the position of a lower tip of an electrode relative to an upper surface of slag is correlated to harmonic frequencies and that if these frequencies are in the range of 100 Hz to 140 Hz the electrode tip must be repositioned. It is not apparent though why signals which lie in this frequency range are considered nor is the nature of the correlation clear.
U.S. Pat. No. 8,410,800 aims to determine “the size and shape” of material but the relevance of this to ascertaining the position of an electrode tip is apparent.
In the applicant's view the various prior art techniques do not clearly define electrical characteristic which is directly associated with an open arc in a furnace.
An object of the present invention is to provide a method of detecting an open arc in a DC arc furnace which is based on identifying a clearly defined electrical characteristic which can be determined from electrical measurements of an external circuit associated with the furnace and which does not require measurements to be taken in an interior of the furnace.